Variable focus liquid lenses are known in the art. Such lenses generally comprise a refractive interface between first and second immiscible liquids that is moveable by electrowetting. FIG. 1 reproduces FIG. 4 of European Patent Application EP1662276, and illustrates a known variable focal length lens 10. Lens 10 comprises two transparent windows 24, 38 facing each other and parallel to one another, and delimiting, at least in part, an internal volume 15 containing two immiscible liquids 16, 18, with different optical indices, defining an optical interface A, B in the form of a meniscus where they meet. The liquids 16, 18 have substantially equal densities, and one is preferably an insulating liquid, for example comprising oil or an oily substance, and the other is preferably a conducting liquid comprising for example an aqueous solution. The windows are for example transparent plates, made of an optical transparent material, e.g. glass.
Lens 10 further comprises a cap 30 connected to transparent window 38 and comprising a first cylindrical side wall 34. Lens 10 also comprises a body 12 having a symmetry of revolution about the optical axis (.DELTA.) of the lens. Body 12 is connected to transparent window 24 and comprises a second cylindrical side wall 20 of a diameter smaller than the diameter of the first cylindrical wall. Cap 30 forms a first electrode and body 12 comprises a second electrode. A gasket 50 is provided to ensure the tightness of the lens structure, positioned such that it is compressed between the first and second cylindrical side walls. In particular, gasket 50 is substantially L-shaped in cross-section, comprising a portion 54 compressed between the first and second cylindrical side walls and a portion 52 compressed between the cap and a top surface 42 of body 12, the top surface 42 comprising an opening defining a conical or cylindrical surface 48 where the interface between the two liquids is able to move.
The lens further comprises deforming portions 36 arranged to deform in response to a change in pressure of the liquids. The deforming portions for example comprise corrugated regions 36 formed in the upper wall 31 of the cap, the deforming portions having symmetry of revolution about the optical axis (Δ) of the lens. For example, the deforming portions comprise at least one circular ripple centred on the optical axis (Δ) of the lens. In this example the cap is for example made of a stamped metal, pressed into shape, e.g. stamped stainless steel. The thickness of the upper wall of the cap will depend on the expected variations of volume to compensate for the effects of expansion of the liquids. For example, a typical thickness of about 0.1 to 0.25 mm has shown good results for lenses whose outer diameters is below 20 mm.
Side wall 34 of the cap comprises a rim 56 crimped onto the body 12, which deforms the gasket 50 between the cap and a corner of body 12, and seals the cap 30 and the body 12. Other methods for sealing the cap onto the body are possible, for example it would be possible to glue the cap 30 onto the body 12.
Due to the electrowetting effect, it is possible, by applying a voltage between the cap 30 and body 16, to change the curvature of the refractive interface between the first liquid 16 and the second liquid 18, for example, from an initial concave shape as shown by dashed line A, to a convex shape as shown by solid line B. Thus rays of light passing through the cell perpendicular to the windows 24, 38 in the region of the refractive interface A, B will be focused more or less depending on the voltage applied.
When variable liquid lens 10 is used in a lens unit, it is generally combined with a number of separate fixed lenses so that the lens unit has the desired total optical power. Such fixed lenses increase the size of the lens unit along the optical axis Δ, which is a disadvantage in some applications where space is limited, such as in compact digital cameras or mobile telephones. The number of fixed lenses that are needed can be reduced if the optical power of variable lens 10 is increased by replacing windows 24 and 38, which are planar windows in the example of FIG. 1, with lenses having a fixed optical power. Such windows would be centred on an optical axis (Δ) of the variable focus lens 10.
However, a problem occurs when mounting such a lens unit to an image sensor. During calibration of the lens unit it is necessary to adjust the positioning of all of the fixed lenses in the lens unit with respect to the image sensor so that images formed on the image sensor are correctly focused. Due to the fixed lenses in the liquid lens, the positioning of the liquid lens must also be adjusted at the same time as the other fixed lenses. However, an electrical connection is required between the electrodes of the liquid lens and driving circuitry that generates the appropriate drive voltages to control the lens. The adjustment of the positioning of the lens unit and in particular of the liquid lens means that it is difficult to make the required electrical connections.